Mitigation of arsenic accumulation in rice with water management and silicon fertilization

Community structure and metabolic potentials of keystone taxa and their associated bacteriophages within rice root endophytic microbiome in response to metal(loid)s contamination

Heavy metal (HM) contamination of agricultural products is of global environmental concern as it directly threatened the food safety. Plant-associated microbiome, particularly endophytic microbiome, hold the potential for mitigating HM stress as well as promoting plant growth. The metabolic potentials of the endophytes, especially those under the HM stresses, have not been well addressed. Rice, a major staple food worldwide, is more vulnerable to HM contamination compared to other crops and therefore requires special attentions. Therefore, this study selected rice as the target plants. Geochemical analysis and amplicon sequencing were combined to characterize the rice root endophytic bacterial communities and identify keystone taxa in two HM-contaminated rice fields. Metagenomic analysis was employed to investigate the metabolic potentials of these keystone taxa. Burkholderiales and Rhizobiales were identified as predominant keystone taxa. The metagenome-assembled genome (MAG)s associated with these keystone populations suggested that they possessed diverse genetic potentials related to metal resistance and transformation (e.g., As resistance and cycling, V reduction, Cr efflux and reduction), and plant growth promotion (nitrogen fixation, phosphate solubilization, oxidative stress resistance, indole-3-acetic acid, and siderophore production). Moreover, bacteriophages encoding auxiliary metabolism genes (AMGs) associated with the HM resistance as well as nitrogen and phosphate acquisition were identified, suggesting that these phages may contribute to these crucial biogeochemical processes within rice roots. The current findings revealed the beneficial roles of rice endophytic keystone taxa and their associated bacteriophages within HM-contaminated rice root endophytic microbiome, which may provide valuable insights on future applications of employing root microbiome for safety management of agriculture productions.

Application of Silica Nanoparticles Induces Different Effects on Inorganic and Methylated Arsenic Accumulation in Rice Grains: Insights from Arsenic Transformation in Soil

Silica nanoparticles (SiNPs) are promising agrochemicals for reducing pollutant accumulation in plants. However, their applications sometimes fail to reduce arsenic in rice grains. Here, we explored the mechanisms using pot experiments via foliar and soil applications, respectively. Both applications significantly decreased As(III) (19.2-35.1%) and DMA (74.4-81.3%) in grains but increased As(V) by 1.2-1.8-fold. SiNP applications reduced As(III) but promoted As(V) in porewater during the flooding period. The As(III)-oxidation gene aioA showed a 1-3 orders of magnitude higher abundance than arsenic-reduction/methylation genes, and its abundance significantly increased after both applications. SiNP applications facilitated the release of Fe-bound organic carbon into porewater and promoted bacterial growth and As(III) oxidation, leading to high As(V) availability for rice uptake. Nevertheless, foliar spraying performed better than soil amendment in reducing total arsenic in grains by inhibiting stem/nodes-to-grain translocation. Our findings highlight the microbial arsenic transformation in soil regulated by SiNPs and demonstrate the advantages of foliar spraying in the arsenic-contaminated paddy field.

Paddy Soil Flooding and Nonflooding Affect the Transcriptional Activity of Arsenic Methylation and Demethylation Communities

The intermediate product methylarsenite [MMA(III)] of arsenic (As) methylation can be methylated to dimethylarsenate (DMA), which causes rice straighthead disease via the As methylation enzyme (ArsM), demethylated to arsenite via the As demethylation enzyme (ArsI), or excreted from cells via the MMA(III) efflux enzyme (ArsP). Whereas As methylation is commonly reported in flooded soils, As demethylation is mostly mediated by aerobes. We used custom-built ROCker models (accuracies of 99.7-99.9%) to quantify the short-read sequences carrying As genes and investigate the variations in the transcriptional activity of the arsM, arsI, and arsP genes in flooded and nonflooded paddy soils. We revealed significantly (p < 0.05) greater transcriptional activity of the arsM and arsP genes in flooded than nonflooded soils, whereas the transcriptional activity of the arsI genes was comparable. MMA(III) demethylation in flooded soils is possibly coupled with denitrification, as revealed by the significantly (p < 0.05) positively correlated genes in terms of transcriptional activity. Moreover, we showed that microbes coexpressing the arsM and arsI genes were dominated by Actinomycetota and Pseudomonadota. This study sheds light on the active microbial communities involved in As methylation and demethylation in paddy soils and provides insights into the prevention of rice straighthead disease.

Rice bran as a potent ingredient: unveiling its potential for value-added applications

Rice bran production significantly contributes to global environmental deterioration, yet its potential remains underutilized. This review discusses the nutritional composition, bioactive compounds, health benefits, limitations, and potential application of rice bran in both food and non-food sectors. While minor variations exist between pigmented and non-pigmented rice bran, the former is abundant in phytochemicals, which offer therapeutic benefits. The primary limitations hindering rice bran's food application include rancidity, toxic heavy metals, and antinutrients. Effective stabilization is crucial to extend rice bran's shelf life. Despite these challenges, rice bran holds significant potential for value-added products. Hence, its rich composition and diverse applications underscore its importance as a valuable resource for sustainable production practices.

Arsenic methylation and microbial communities in paddy soils under alternating anoxic and oxic conditions

Arsenic (As) methylation in soils affects the environmental behavior of As, excessive accumulation of dimethylarsenate (DMA) in rice plants leads to straighthead disease and a serious drop in crop yield. Understanding the mobility and transformation of methylated arsenic in redox-changing paddy fields is crucial for food security. Here, soils including un-arsenic contaminated (N-As), low-arsenic (L-As), medium-arsenic (M-As), and high-arsenic (H-As) soils were incubated under continuous anoxic, continuous oxic, and consecutive anoxic/oxic treatments respectively, to profile arsenic methylating process and microbial species involved in the As cycle. Under anoxic-oxic (A-O) treatment, methylated arsenic was significantly increased once oxygen was introduced into the incubation system. The methylated arsenic concentrations were up to 2-24 times higher than those in anoxic (A), oxic (O), and oxic-anoxic (O-A) treatments, under which arsenic was methylated slightly and then decreased in all four As concentration soils. In fact, the most plentiful arsenite S-adenosylmethionine methyltransferase genes (arsM) contributed to the increase in As methylation. Proteobacteria (40.8%-62.4%), Firmicutes (3.5%-15.7%), and Desulfobacterota (5.3%-13.3%) were the major microorganisms related to this process. These microbial increased markedly and played more important roles after oxygen was introduced, indicating that they were potential keystone microbial groups for As methylation in the alternating anoxic (flooding) and oxic (drainage) environment. The novel findings provided new insights into the reoxidation-driven arsenic methylation processes and the model could be used for further risk estimation in periodically flooded paddy fields.

Boosting Rice Resilience: Role of Biogenic Nanosilica in Reducing Arsenic Toxicity and Defending against Herbivore

The use of nanoparticles is a promising ecofriendly strategy for mitigating both abiotic and biotic stresses. However, the physiological and defense response mechanisms of plants exposed to multiple stresses remain largely unexplored. Herein, we examined how foliar application of biogenic nanosilica (BNS) impacts rice plant growth, molecular defenses, and metabolic responses when subjected to arsenic (As) toxicity and infested by the insect Chilo suppressalis. We show that BNS significantly increased shoot and root silicon accumulation but reduced the shoot As content by 34.7% under herbivory. Additionally, BNS reduced C. suppressalis larval weight gain by 34.5 and 12.3% without and with As stress, respectively. Importantly, BNS enhanced antioxidant enzyme activity under As stress, herbivore attack, and combined pressures, surpassing the effects of traditional silicate fertilizers. BNS ultimately increased rice shoot biomass by 8.2-23.4% under the respective stress conditions compared to the control treatment. Moreover, while As stress alone diminished the plant's resistance to herbivores, BNS application countered this effect by increasing detoxifying compound (e.g., glutathione) production and antioxidant enzyme activity. This study highlights the impact of biotic and abiotic stress interactions on BNS-enhanced plant resilience mechanisms in rice plants, reallocating resources to counter heavy metal toxicity and herbivore damage in agroecosystems.

Contrasting effects of arsenic on mycorrhizal-mediated silicon and phosphorus uptake by rice

Silicon (Si) and arbuscular mycorrhizal fungi (AMF) increase plant resistance to various environmental stresses, including heavy metal (and metalloid) toxicity. Although Si and AMF each independently enhance plant tolerance, the nature of their interactions and their combined impacts on nutrient uptake, especially in the context of toxic elements such as arsenic (As), remains to be elucidated. This study investigated AMF-mediated regulation of plant nutrient uptake under As stress using rice, a model Si-accumulating plant. Experiments were conducted under As-free and As stress conditions, incorporating AMF inoculation and silicic acid application, with a focus on nutrient uptake and transporter expression. Without As, AMF inoculation increased shoot Si content by 44%, while invariance was common under As toxicity stress (10 μM of As(III)). Despite As presence, AMF increased Lsi1 expression with Si application, elevating As content in roots and shoots by 38% and 55%, respectively. Introduction of As stress amplified AMF role in phosphorus (P) uptake from 13% to 38%, correlating with up-regulated P transporter expression. Three-way ANOVA of interactions among As, Si, and AMF on P and As uptake by rice revealed that As amplified AMF potential to increase P uptake while weakening promotive effect on Si uptake. Silicon reduced As absorption, while AMF increased As uptake, but the elevated As were potentially retained within fungal hyphae, limiting transfer to rice plants. Overall, As toxicity stress had contrasting effects on P- and Si-promoting roles of AMF. These findings contribute to our understanding of plant-fungal interactions under heavy metal stress.

Coupled reduction in arsenic methylation and methanogenesis with silicate amendment in arsenic-enriched paddy soils

Methanogens play an important role in the demethylation of arsenic. Soil amendments that inhibit methanogens can increase dimethylarsinic acid (DMA), which is responsible for straighthead disease in rice. A decrease in methanogenesis caused by silicate fertilizer may increase DMA concentration in paddy soils and rice grains; the relationship between these two factors and their impacts on DMA concentration remains unclear. We applied silicate fertilizer (2 Mg ha-1) to japonica and indica rice grown on arsenic-spiked soils and found a simultaneous reduction in methane emissions and pore-water DMA concentration, compared to no-silicate fertilization. Gene and transcript copies of mcrA and arsM, as well as dominant methanogens and arsenic-methylating microbes decreased significantly with silicate fertilization. However, the sulfate-reducing bacteria and the gene and transcript copies of dsrB did not change significantly in response to the application of silicate fertilizer to paddy soils. The abundance of arsenic methylating microbes was significantly and positively correlated with the abundance of methanogens, but not with the abundance of sulfate-reducing bacteria. Methylomonas and Methylobacter, which harbor the arsM gene, were suppressed under silicate fertilization, suggesting that they have the potential to methylate As and play a crucial role in reducing pore-water DMA in As-enriched flooded paddy soils. Increasing Fe concentration, soil pH, and Eh value decreased pore-water DMA concentration, while decreasing arsenite concentration, arsM and mcrA gene abundance decreased it. While silicate fertilization decreased arsenite and DMA concentrations in pore-water, it had no significant effect on rice DMA content, but significantly decreased arsenite content. Results reveal that methanogens and arsenic-methylating microbes have a synergistic relationship under silicate fertilization that facilitates a significant reduction in methane emissions and DMA concentration in As-enriched paddy soils.

Midseason draining of paddy water suppresses microbial arsenic methylation in soil and alleviates rice straighthead disease

Arsenic (As) methylation is an important component of As biogeochemical cycle. Microbial As methylation is enhanced under anoxic conditions in paddy soil, producing dimethylarsenate (DMA) which can cause physiological straighthead disease in rice. We conducted field experiments at two sites to test the effect of midseason draining of paddy water on microbial As methylation and the incidence of straighthead disease. Compared to continuous flooding, midseason draining increased soil Eh, decreased the abundances of microbial genes for arsenate reduction (arsC and arrA) and arsenite methylation (arsM), and lowered the concentrations of both inorganic As and DMA in soil porewater. Draining shifted microbial composition, resulting in decreases in the relative abundance of 17-132 amplicon sequence variants. Draining decreased the accumulation of DMA in rice husk and of inorganic As and DMA in rice grain, decreased the incidence of straighthead disease, and increased grain yield by 20-45 %. Further experiments were conducted at eight field sites to assess the effect of midseason draining in a split field design. Draining decreased husk DMA concentration by 40-65 % and increased grain yield by 25-209 %. This study demonstrates that midseason draining can effectively suppress microbial As methylation and alleviate rice straighthead disease, benefiting both grain yield and safety.

Phase transformation of schwertmannite in paddy soil under different water management regimes and its impact on the migration of arsenic in soil

Schwertmannite (Sch) holds a great promise as an iron material for remediating Arsenic (As)-contaminated paddy soils, due to its extremely high immobilization capacities for both arsenate [As(V)] and arsenite [As(III)]. However, there is still limited knowledge on the mineral phase transformation of this metastable iron-oxyhydroxysulfate mineral in paddy soils, particularly under different water management regimes including aerobic, intermittent flooding, and continuous flooding, and how its phase transformation impacts the migration of As in paddy soils. In this study, a membrane coated with schwertmannite was first developed to directly reflect the phase transformation of bulk schwertmannite applied to paddy soils. A soil incubation experiment was then conducted to investigate the mineral phase transformation of schwertmannite in paddy soils under different water management regimes and its impact on the migration of As in paddy soil. Our findings revealed that schwertmannite can persist in the paddy soil for 90 days in the aerobic group, whereas in the continuous flooding and intermittent flooding groups, schwertmannite transformed into goethite, with the degree or rate of mineral phase transformation being 5% Sch >1% Sch > control. These results indicated that water management practices and the amount of schwertmannite applied were the primary factors determining the occurrence and degree of mineral transformation of schwertmannite in paddy soil. Moreover, despite undergoing phase transformation, schwertmannite still significantly reduced the porewater As (As(III) and As(V)), and facilitated the transfer of non-specifically adsorbed As (F1) and specifically adsorbed As (F2) to amorphous iron oxide-bound As (F3), effectively reducing the bioavailability of soil As. These findings contribute to a better understanding of the mineralogical transformation of schwertmannite in paddy soils and the impact of mineral phase transformation on the retention of As in soil, which carry important implications for the application of schwertmannite in remediating As-contaminated paddy soils.

Unintended food safety impacts of agricultural circular economies, with case studies in arsenic and mycotoxins

For millennia, food systems worldwide have employed practices befitting a circular economy: recycling of agricultural and food waste or byproducts, environmentally sustainable production methods, and food preservation to reduce waste. Many modern-day agricultural practices may also contribute to a circular economy through the reuse of waste products and/or reducing agricultural inputs. There are, however, food safety impacts. This paper describes two sustainable agricultural practices that have unintended positive and negative impacts on food safety: alternative rice cultivation practices and no-till agriculture. We highlight how alternative rice cultivation practices have intended benefits of water conservation and economic savings, yet also unintended effects on food safety by reducing foodborne arsenic levels while increasing cadmium levels. No-till agriculture reduces soil erosion and repurposes crop residues, but can lead to increased foodborne mycotoxin levels. Trade-offs, future research, and policy recommendations are discussed as we explore the duality of sustainable agricultural practices and food safety.

Controlling exposure to As and Cd from rice via irrigation management

Irrigation management controls biogeochemical cycles in rice production. Under flooded paddy conditions, arsenic becomes plant-available as iron-reducing conditions ensue, while oxic conditions lead to increased plant availability of Cd in acidic soils. Because Cd enters rice through Mn transporters, we hypothesized that irrigation resulting in intermediate redox could simultaneously limit both As and Cd in rice grain due to As retention in soil and Mn competition for Cd uptake. In a 2 year field study, we used 6 irrigation managements that varied in extent and frequency of inundation, and we observed strong effects of irrigation management on porewater chemistry, soil redox potentials, plant As and Cd concentrations, plant nutrient concentrations, and methane emissions. Plant As decreased with drier irrigation management, but in the grain this effect was stronger for organic As than for inorganic As. Grain organic As, but not inorganic As, was strongly and positively correlated with cumulative methane emissions. Conversely, plant Cd increased under more aerobic irrigation management and grain Cd was negatively correlated with porewater Mn. A hazard index approach showed that in the tested soil with low levels of As and Cd (5.4 and 0.072 mg/kg, respectively), irrigation management could not simultaneously decrease grain As and Cd. Many soil properties, such as reducible As, available Cd, soil pH, available S, and soil organic matter should be considered when attempting to optimize irrigation management when the goal is decreasing the risk of As and Cd in rice grain.

Effectiveness of nanomaterials and their counterparts in improving rice growth and yield under arsenic contamination

Arsenic (As) pollution in rice (Oryza sativa L.), a staple food for over 3.5 billion people, is a global problem. Mixed effects of Zn, Cu, and Si amendments on plant growth and yield, including in the presence of As pollution have been reported in previous studies. To better investigate the effectiveness of these amendments on rice growth, yield, and As accumulation, we conducted a rice greenhouse experiment with 11 treatments, including control pots with and without As contamination and pots with amendments of ZnO, CuO, and SiO2 nanoparticles (ZnO NPs, CuO NPs, and SiO2 NPs), their ionic counterparts (ZnSO4, CuSO4, and Na2SiO3), and bulk particles (ZnO BPs, CuO BPs, and SiO2 BPs). Compared with the background soil, the treatment of adding As decreased rice plant height, panicle number, and grain yield by 16.5%, 50%, and 85.7%, respectively, but significantly increased the As accumulation in milled rice grains by 3.2 times. Under As contamination, the application of Zn amendments increased rice grain yield by 4.6-7.3 times; among the three Zn amendments, ZnSO4 performed best by fully recovering grain yield to the background level and significantly reducing grain AsIII/total As ratio by 46.9%. Under As contamination, the application of Cu amendments increased grain yield by 3.8-5.6 times; all three Cu amendments significantly reduced grain AsIII/total As ratio by 20.2-65.6%. The results reveal that Zn and Cu amendments could promote rice yield and prevent As accumulation in rice grains under As contamination. Despite the observed reduction in As toxicity by the tested NPs, they do not offer more advantages over their ionic counterparts and bulk particles in promoting rice growth under As contamination. Future field research using a broader range of rice varieties, investigating various As concentrations, and encompassing diverse climate conditions will be necessary to validate our findings in achieving more extensive understanding of effective management of arsenic contaminated rice field.

Understanding phloem's role in long-distance transport and accumulation of arsenic (As) in rice: toward low-As-accumulating grain development

MAIN CONCLUSION

This review highlights the roles of phloem in the long-distance transport and accumulation of As in rice plants, facilitating the formulation of new strategies to reduce the grain As content. Rice is a staple diet for a significant proportion of the global population. As toxicity is a major issue affecting the rice productivity and quality worldwide. Phloem tissues of rice plants play vital roles in As speciation, long-distance transport, and unloading, thereby controlling the As accumulation in rice grains. Phloem transport accounts for a significant proportion of As transport to grains, ranging from 54 to 100% depending on the species [inorganic arsenate (As(V)), arsenite (As(III)), or organic dimethylarsinic acid (DMA(V)]. However, the specific mechanism of As transport through phloem leading to its accumulation in grains remains unknown. Therefore, understanding the molecular mechanism of phloem-mediated As transport is necessary to determine the roles of phloem in long-distance As transport and subsequently reduce the grain As content via biotechnological interventions. This review discusses the roles of phloem tissues in the long-distance transport and accumulation of As in rice grains. This review also highlights the biotechnological approaches using critical genetic factors involved in nodal accumulation, vacuolar sequestration, and cellular efflux of As in phloem- or phloem-associated tissues. Furthermore, the limitations of existing transgenic techniques are outlined to facilitate the formulation of novel strategies for the development of rice with reduced grain As content.

The Reaction of Rice Growth to the Arsenic Contamination under Various Irrigation Methods

Several studies have explored how arsenic (As) is absorbed and transported in plants, but less attention has been paid to its impact on rice growth and yield in relation to water management. We aimed to assess how arsenic affects plant development under different irrigation methods. The growth and yield parameters of four rice varieties ('M 488', 'Janka', 'Szellő', and 'Nembo') in two greenhouse experiments were analyzed in 2021 and 2022 under different irrigation methods (flooded (F), intermittent (I), and aerobic (A)). Three different As concentrations were set up in the soil: 43 mg kg-1, 24 mg kg-1, and 4 mg kg-1. Our results showed that the high As treatment caused severe damage to the plants including leaf yellowing as well as reduced growth and decreased yield parameters. Alternative water management practices such as I and A irrigation could reduce the negative effects of As. At the high level of As stress (43 mg kg-1), the I irrigation had the most favorable effect on the yield of 'Janka' among the tested cultivars compared to the F irrigation (in F: 1.64 ± 1.13 g; in I: 5.45 ± 3.69 g). However, the use of fully aerobic conditions increased the likelihood of drought stress.

Genetic engineering low-arsenic and low-cadmium rice grain

Rice is prone to take up the toxic elements arsenic (As) and cadmium (Cd) from paddy soil through the transporters for other essential elements. Disruption of these essential transporters usually adversely affects the normal growth of rice and the homeostasis of essential elements. Here we report on developing low-As and low-Cd rice grain through the co-overexpression of OsPCS1, OsABCC1, and OsHMA3 genes under the control of the rice OsActin1 promoter. Co-overexpression of OsPCS1 and OsABCC1 synergistically decreased As concentration in the grain. Overexpression of OsPCS1 also decreased Cd concentration in the grain by restricting the xylem-to-phloem Cd transport in node I, but paradoxically caused Cd hypersensitivity as the overproduced phytochelatins in OsPCS1-overexpressing plants suppressed OsHMA3-dependent Cd sequestration in vacuoles and promoted Cd transport from root to shoot. Co-overexpression of OsHAM3 and OsPCS1 overcame this suppression and complemented the Cd hypersensitivity. Compared with non-transgenic rice control, co-overexpression of OsABCC1, OsPCS1, and OsHMA3 in rice decreased As and Cd concentrations in grain by 92.1% and 98%, respectively, without causing any defect in plant growth and reproduction or of mineral nutrients in grain. Our research provides an effective approach and useful genetic materials for developing low-As and low-Cd rice grain.

Temporal development of arsenic speciation and extractability in acidified and non-acidified paddy soil amended with silicon-rich fly ash and manganese- or zinc-oxides under flooded and drainage conditions

Oxides of silicon (Si), manganese (Mn), and zinc (Zn) have been used as soil amendments to reduce As mobility and uptake in paddy soil systems. However, these amendments are hypothesized to be affected differently depending on the soil pH and their effect on As speciation in rice paddy systems is not fully understood. Herein, we used a microcosm experiment to investigate the effects of natural Si-rich fly ash and synthetic Mn and Zn oxides on the temporal development of porewater chemistry, including aqueous As speciation (As(III), As(V), MMA, DMA, and DMMTA) and solid-phase As solubility, in a naturally calcareous soil with or without soil acidification (with sulfuric acid) during 28 days of flooding and subsequent 14 days of drainage. We found that soil acidification to pH 4.5 considerably increased the solubility of Si, Fe, Mn, and Zn compared to the non-acidified soil. Additions of Mn and Zn oxides decreased the concentrations of dissolved arsenite and arsenate in the non-acidified soil whereas additions of Zn oxide and combined Si-Zn oxides increased them in the acidified soil. The Si-rich fly ash did not increase dissolved Si and As in the acidified and non-acidified soils. Dimethylated monothioarsenate (DMMTA) was mainly observed in the acidified soil during the later stage of soil flooding. The initial 28 days of soil flooding decreased the levels of soluble and exchangeable As and increased As associated with Mn oxides, whereas the subsequent 14 days of soil drainage reversed the trend. This study highlighted that soil acidification considerably controlled the solubilization of Ca and Fe, thus influencing the soil pH-Eh buffering capacity, the solubility of Si, Mn, and Zn oxides, and the mobility of different As species in carbonate-rich and acidic soils under redox fluctuations.

Arsenic (As) accumulation in different genotypes of indica rice (Oryza sativa L.) and health risk assessment based on inorganic As

To reveal differences in arsenic (As) accumulation among indica rice cultivars and assess the human health risks arising from inorganic arsenic (iAs) intake via rice consumption, a total of 320 field indica rice samples and corresponding soil samples were collected from Fujian Province in China. The results showed that available soil As (0.03 to 3.83 mg/kg) showed a statistically significant positive correlation with total soil As (0.10 to 19.45 mg/kg). The inorganic As content in brown rice was between 0.001 and 0.316 mg/kg. Among the cultivars, ten brown rice samples (3.13%) exceeded the maximum contaminant level (MCL) of iAs in food of 0.2 mg/kg in China. The estimated daily intake (EDI) and calculated individual incremental lifetime cancer risk (ILCR) ranged from 0.337 µg/day to 106.60 µg/day and from 8.18 × 10-6 to 2.59 × 10-3, respectively. Surprisingly, the average EDI and the EDIs of 258 (80.63%) brown rice samples were higher than the maximum daily intake (MDI) of 10 µg/day in drinking water as set by the National Research Council. The mean ILCR associated with iAs was 54.3 per 100,000, which exceeds the acceptable upper limit (AUL) of 10 per 100,000 set by the USEPA. Notably, the cultivars Y-Liang-You 1 and Shi-Ji 137 exhibited significantly higher mean ILCRs compared to the AUL and other cultivars, indicating that they pose more serious cancer risks to the local population. Finally, this study demonstrated that the cultivars Yi-Xiang 2292 and Quan-Zhen 10 were the optimal cultivars to mitigate risks associated with iAs to human health from rice consumption.

How Fe-bearing materials affect soil arsenic bioavailability to rice: A meta-analysis

Arsenic (As) contamination is widespread in soil and poses a threat to agricultural products and human health due to its high susceptibility to absorption by rice. Fe-bearing materials (Fe-Mat) display significant potential for reducing As bioavailability in soil and bioaccumulation in rice. However, the remediation effect of various Fe-Mat is often inconsistent, and the response to diverse environmental factors is ambiguous. Here, we conducted a meta-analysis to quantitatively assess the effects of As in soils, rice roots, and grains based on 673, 321, and 305 individual observations from 67 peer-reviewed articles, respectively. On average, Fe-Mat reduced As bioavailability in soils, rice roots, and grains by 28.74 %, 33.48 %, and 44.61 %, respectively. According to the analysis of influencing factors, the remediation efficiency of Fe-Mat on As-contaminated soil was significantly enhanced with increasing Fe content in the material, in which the industry byproduct was the most effective in soils (-42.31 %) and rice roots (-44.57 %), while Fe-biochar was superior in rice grains (-54.62 %). The efficiency of Fe-Mat in minimizing soil As mobility was negatively correlated with soil Fe content, CEC, and pH. In addition, applying Fe-Mat in alkaline soils with higher silt, lower clay and available P was more effective in reducing As in rice grains. A higher efficiency of applying Fe-Mat under continuous flooding conditions (27.39 %) compared with alternate wetting and drying conditions (23.66 %) was also identified. Our results offer an important reference for the development of remediation strategies and methods for various As-contaminated paddy soils.

Unveiling the significance of foliar-applied silicon, selenium and phosphorus for the management and remediation of arsenic in two different rice genotypes

Under paddy soil conditions, rice plants are vulnerable to arsenic (As) accumulation, thus causing potential threat to human health. Here we investigated the influence of foliar-applied phosphorus (P: 10 and 20 mg L-1), silicon (Si: 0.6 and 1.5 g L-1) and selenium (Se: 5 and 10 mg L-1) on As accumulation, morphological and physiological attributes of two contrasting rice genotypes (KSK-133 and Super Basmati) under As stress (25 mg kg-1 as arsenate). Silicon foliar dressing significantly (p < 0.05) reduced grain As uptake (up to 67%) and improved rice growth and chlorophyll content (28-66%) in both rice genotypes over their controls. Phosphorus foliar application resulted in a notable decrease (17%) in grain As uptake of coarse rice genotype (KSK-133), while it slightly increased grain As uptake in the fine one (Super Basmati; 6%) compared to controls. However, foliar-applied Se did not show significant effects on rice plants growth attributes and As uptake in both genotypes. Similarly, biochemical and enzymatic attributes (i.e., lipid peroxidation, electrolyte leakage, peroxidase and catalase) were improved with Si application in rice plants, except for P treatment that was only effective for coarse one. Foliar-applied Si also resulted in reduced cancer risk and hazard quotient (< 0.10) for both rice genotypes. This study advances our understanding on critical role of different foliar-applied nutrients and rice genotypes, which is imperative to develop effective As remediation and management strategies in coarse and fine rice genotypes and protect human health.

Translocation, enzymatic reduction and toxicity of dimethylarsenate in rice

Dimethylarsenate [DMAs(V)] can be produced by some soil microorganisms through methylation of inorganic arsenic (As), especially in anoxic paddy soils. DMAs(V) is more phytotoxic than inorganic As and can cause the physiological disorder straighthead disease in rice. Rice cultivars vary widely in the resistance to DMAs(V), but the mechanism remains elusive. Here, we investigated the differences in DMAs(V) uptake, translocation, and reduction to dimethylarsenite [DMAs(III)], as well as the effects on the metabolome, between two rice cultivars Mars and Zhe733. We found that Mars was 11-times more resistant to DMAs(V) than Zhe733. Mars accumulated more DMAs(V) in the roots, whereas Zhe733 translocated more DMAs(V) to the shoots and reduced more DMAs(V) to DMAs(III). DMAs(III) was more toxic than DMAs(V). Using heterologous expression and in vitro enzyme assays, we showed that the glutathione-S-transferases OsGSTU17 and OsGSTU50 were able to reduce DMAs(V) to DMAs(III). The expression levels of OsGSTU17 and OsGSTU50 were higher in the shoot of Zhe733 compared to Mars. Metabolomic analysis in rice shoots showed that glutathione (GSH) metabolism was perturbed by DMAs(V) toxicity in Zhe733. Application of exogenous GSH significantly alleviated the toxicity of DMAs(V) in Zhe733. Taken together, the results suggest that Mars is more resistant to DMAs(V) than Zhe733 because of a lower root-to-shoot translocation and a smaller capacity to reduce DMAs(V) to DMAs(III).

Editing Silicon Transporter Genes to Reduce Arsenic Accumulation in Rice

Rice is a dominant source of inorganic arsenic (As) exposure for populations consuming rice as a staple food. Decreasing As accumulation in rice grain is important for improving food safety. Arsenite [As(III)], the main form of As in paddy soil porewater, is taken up inadvertently by OsLsi1 and OsLsi2, the two key transporters for silicon (Si) uptake in rice roots. Here, we investigated whether editing OsLsi1 or OsLsi2 can decrease As accumulation in rice grain without compromising grain yield. We used the CRISPR-Cas9 technology to edit the promoter region of OsLsi1 and the C-terminal coding sequence of OsLsi1 and OsLsi2, and we generated a total of 27 mutants. Uptake and accumulation of Si and As were evaluated in both short-term hydroponic experiments and in a paddy field. Deletion of 1.2-2 kb of the OsLsi1 promoter suppressed OsLsi1 expression in roots and Si uptake markedly and did not affect As(III) uptake or grain As concentration. Some of the OsLsi1 and OsLsi2 coding sequence mutants showed large decreases in the uptake of Si and As(III) as well as large decreases in Si accumulation in rice husks. However, only OsLsi2 mutants showed significant decreases (by up to 63%) in the grain total As concentration. Editing OsLsi2 mainly affected the accumulation of inorganic As in rice grain with little effect on the accumulation of dimethylarsenate (DMA). Grain yields of the OsLsi2 mutants were comparable to those of the wild type. Editing OsLsi2 provides a promising way to reduce As accumulation in rice grain without compromising the grain yield.

Heavy Metals in Agricultural Soils: Sources, Influencing Factors, and Remediation Strategies

Soil heavy metal pollution is a global environmental challenge, posing significant threats to eco-environment, agricultural development, and human health. In recent years, advanced and effective remediation strategies for heavy metal-contaminated soils have developed rapidly, and a systematical summarization of this progress is important. In this review paper, first, the anthropogenic sources of heavy metals in agricultural soils, including atmospheric deposition, animal manure, mineral fertilizers, and pesticides, are summarized. Second, the accumulation of heavy metals in crops as influenced by the plant characteristics and soil factors is analyzed. Then, the reducing strategies, including low-metal cultivar selection/breeding, physiological blocking, water management, and soil amendment are evaluated. Finally, the phytoremediation in terms of remediation efficiency and applicability is discussed. Therefore, this review provides helpful guidance for better selection and development of the control/remediation technologies for heavy metal-contaminated agricultural soils.

The application of mixed stabilizing materials promotes the feasibility of the intercropping system of Gynostemma pentaphyllum/Helianthus annuus L. on arsenic contaminated soil

Intercropping technology and stabilizing materials are common remediation techniques for soils contaminated with heavy metals. This study investigated the feasibility of the Gynostemma pentaphyllum (G. pentaphyllum)/Helianthus annuus L. (H. annuus) intercropping system on arsenic (As) contaminated farmland through field and pot experiments and the regulation of plant As absorption by the application of mixed stabilizing materials in this intercropping system. Field experiments demonstrated that intercropping with H. annuus increased the As concentration in G. pentaphyllum leaves to 1.79 mg kg-1 but still met the requirements of the national food standard of China (2 mg kg-1) (GB2762-2017). Meanwhile, G. pentaphyllum yield in the intercropping system decreased by 15.09%, but the difference was insignificant (P > 0.05). Additionally, the As bioconcentration (BCA) per H. annuus plant in the intercropping system was significantly higher than that in the monoculture system, increasing by 76.37% (P < 0.05). The pot experiment demonstrated that when granite powder, iron sulfate mineral, and "Weidikang" soil conditioner were applied to the soil collectively, G. pentaphyllum leaf As concentration in the intercropping system could be significantly reduced by 42.17%. Rhizosphere pH is the most crucial factor affecting As absorption by G. pentaphyllum in intercropping systems. When these three stabilizing materials were applied simultaneously, the As bioaccumulation (BCA) per H. annuus plant was significantly higher than that of normal intercropping treatment, which increased by 71.12% (P < 0.05), indicating that the application of these stabilizing materials significantly improved the As removal efficiency of the intercropping system. Dissolved organic carbon (DOC) concentration in the rhizosphere soil is the most pivotal factor affecting As absorption by H. annuus. In summary, the G. pentaphyllum-H. annuus intercropping model is worthy of being promoted in moderately As polluted farmland. The application of granite powder, iron sulfate mineral, and "Weidikang" soil conditioner collectively to the soil can effectively enhance the potential of this intercropping model to achieve "production while repairing" in the As polluted farmland.

Exploring the potential: Can arsenic (As) resistant silicate-solubilizing bacteria manage the dual effects of silicon on As accumulation in rice?

Rice (Oryza sativa L.) cultivation in regions marked by elevated arsenic (As) concentrations poses significant health concerns due to As uptake by the plant and its subsequent entry into the human food chain. With rice serving as a staple crop for a substantial share of the global population, addressing this issue is critical for food security. In flooded paddy soils, where As availability is pronounced, innovative strategies to reduce As uptake and enhance agricultural sustainability are mandatory. Silicon (Si) and Si nanoparticles have emerged as potential candidates to mitigate As accumulation in rice. However, their effects on As uptake exhibit complexity, influenced by initial Si levels in the soil and the amount of Si introduced through fertilization. While low Si additions may inadvertently increase As uptake, higher Si concentrations may alleviate As uptake and toxicity. The interplay among existing Si and As availability, Si supplementation, and soil biogeochemistry collectively shapes the outcome. Adding water-soluble Si fertilizers (e.g., Na2SiO3 and K2SiO3) has demonstrated efficacy in mitigating As toxicity stress in rice. Nonetheless, the expense associated with these fertilizers underscores the necessity for low cost innovative solutions. Silicate-solubilizing bacteria (SSB) resilient to As hold promise by enhancing Si availability by accelerating mineral dissolution within the rhizosphere, thereby regulating the Si biogeochemical cycle in paddy soils. Promoting SSB could make cost-effective Si sources more soluble and, consequently, managing the intricate interplay of Si's dual effects on As accumulation in rice. This review paper offers a comprehensive exploration of Si's nuanced role in modulating As uptake by rice, emphasizing the potential synergy between As-resistant SSB and Si availability enhancement. By shedding light on this interplay, we aspire to shed light on an innovative attempt for reducing As accumulation in rice while advancing agricultural sustainability.

Enhanced As extraction from paddy soils with high As contamination risk by rice plant upon Si fertilization

Owing to flooded growing conditions and specific physiological characteristics, rice plant is more efficient in As uptake and accumulation, which provides a cost-effective and time-efficient pathway to deplete bioavailable As from paddy soils. In the present study, the enhancing effect of silicon (Si) fertilization on As extraction from heavily contaminated paddy soils by rice was explored Upon incorporation of one weak acid Si fertilizer (AcSF), soil As solubility was significantly promoted by 1.3-1.4-fold, while a slightly increase in porewater As was observed with alkaline soluble Si fertilizer Na2SiO3 (AlSF). With both Si fertilizers applied before transplanting, a relatively low Si/As molar ratio (<100) in soil porewater was obtained, As a result, soil As uptake by rice plant with Si fertilizers was enhanced by 37.2%-171.7% compared to control (CK). Notably, up to 91.6% of the total As in rice plant retained in root with Si fertilization, suggesting the importance of root removal. By harvesting the whole rice plant including roots, soil bioavailable As measured by diffusive gradients in thin films (DGT) declined by 26.9%-31.3% in AlSF treatments relative to CK. Total soil As depletion by the whole rice plant was significantly enhanced from 2.8% in CK to 7.0%-11.2% in Si fertilizer treatments. In this way, 197.5 mg As m-2-232.5 mg As m-2 could be eliminated from soil following one rice-growth season, which was 2.3-2.7-fold higher compared to CK. These results identified the effectiveness of soluble Si fertilizer in enhancing soil As depletion by rice from paddy soils with high As contamination risk, which could serve as a cost-effective strategy with little technical-restriction.

Methylotrophic methanogens and bacteria synergistically demethylate dimethylarsenate in paddy soil and alleviate rice straighthead disease

Microorganisms play a key role in arsenic (As) biogeochemistry, transforming As species between inorganic and organic forms and different oxidation states. Microbial As methylation is enhanced in anoxic paddy soil, producing primarily dimethylarsenic (DMAs), which can cause rice straighthead disease and large yield losses. DMAs can also be demethylated in paddy soil, but the microorganisms driving this process remain unclear. In this study, we showed that the enrichment culture of methylotrophic methanogens from paddy soil demethylated pentavalent DMAs(V) efficiently. DMAs(V) was reduced to DMAs(III) before demethylation. 16S rRNA gene diversity and metagenomic analysis showed that Methanomassiliicoccus dominated in the enrichment culture, with Methanosarcina and Methanoculleus also being present. We isolated Methanomassiliicoccus luminyensis CZDD1 and Methanosarcina mazei CZ1 from the enrichment culture; the former could partially demethylate trivalent DMAs(III) but not DMAs(V) and the latter could demethylate neither. Addition of strain CZDD1 to the enrichment culture greatly accelerated DMAs(V) demethylation. Demethylation of DMAs(V) in the enrichment culture was suppressed by ampicillin, suggesting the involvement of bacteria. We isolated three anaerobic bacterial strains including Clostridium from the enrichment culture, which could produce hydrogen and reduce DMAs(V) to DMAs(III). Furthermore, augmentation of the Methanomassiliicoccus-Clostridium coculture to a paddy soil decreased DMAs accumulation by rice and alleviated straighthead disease. The results reveal a synergistic relationship whereby anaerobic bacteria reduce DMAs(V) to DMAs(III) for demethylation by Methanomassiliicoccus and also produce hydrogen to promote the growth of Methanomassiliicoccus; enhancing their populations in paddy soil can help alleviate rice straighthead disease.

Optimizing directional recovery of high-bioavailable phosphorus from human manure: Molecular-level understanding and assessment of application potential

Phosphorus (P) recovery from human manure (HM) is critical for food production security. For the first time, a one-step hydrothermal carbonation (HTC) treatment of HM was proposed in this study for the targeted high-bioavailable P recovery from P-rich hydrochars (PHCs) for direct soil application. Furthermore, the mechanism for the transformation of P speciation in the derived PHCs was also studied at the molecular level. A high portion of P (80.1∼89.3%) was retained in the solid phase after HTC treatment (120∼240°C) due to high metal contents. The decomposition of organophosphorus (OP) into high-bioavailable orthophosphate (Ortho-P) was accelerated when the HTC temperature was increased, reaching ∼97.1% at 210°C. In addition, due to the high content of Ca (40.45±2.37 g/kg) in HM, the HTC process promoted the conversion of low-bioavailable non-apatite inorganic (NAIP) into high-bioavailable apatite inorganic P (AP). In pot experiments with pea seedling growth, the application of newly obtained PHCs significantly promoted plant growth, including average wet/dry weight and plant height. Producing 1 ton of PHCs (210°C) with the same effective P content as agricultural-type calcium superphosphate could result in a net return of $58.69. More importantly, this pathway for P recovery is predicted to meet ∼38% of the current agricultural demand.

The protective role of tetraploidy and nanoparticles in arsenic-stressed rice: Evidence from RNA sequencing, ultrastructural and physiological studies

Genome doubling in plants induces physiological and molecular changes to withstand environmental stress. Diploid rice (D-2x) and its tetraploid (T-4x) plants were treated with 25 μM Arsenic (As) and 15 mg L-1 TiO2 nanoparticles (NPs), and results indicated decreased growth and photosynthetic activity with high accumulation of reactive oxygen species (ROS) due to the As-toxicity in rice lines, significantly in D-2x rice plants. The treatment of As-contaminated rice with TiO2 NPs resulted in increased root length (8.17%) and chlorophyll AB (13.28%) and decreased electrolyte leakage (21.76%) and H2O2 (17.65%) contents than its counterpart diploid rice. Moreover, TiO2 NPs improved the activity of peroxidase, catalase, glutathione, and superoxide dismutase and reduced lipid peroxidation due to lower ROS production in D-2x and T-4x under As toxicity. Transcriptome analysis revealed abrupt changes in the expression levels of key signaling heat shock proteins, tubulin, aquaporins, As, and metal transporters under As toxicity in T-4x and D-2x lines. The KEGG and GO studies highlighted the striking distinctions between rice lines under As-stress in glutathione metabolism, H2O2 catabolic process, MAPK signaling pathway, and carotenoid biosynthesis terms, revealing consistency between physiological and molecular results. Root cells from D-2x rice were significantly more distorted by As poisoning than those from 4x rice, and cell organelles, such as mitochondria and endoplasmic reticulum, were changed or deformed. These findings proved the superiority of tetraploid rice lines over their diploid counterpart in coping with As-stress.

Synergistic impacts of ferromanganese oxide biochar and optimized water management on reducing Cd accumulation in rice

Ferromanganese oxide biochar composite (FMBC) is an efficient remediation material for cadmium -contaminated soils. However, the effect of FMBC under varied water managements on the remediation of Cd-polluted soil is unclear. In this study, we conducted both incubation and field experiments to investigate the combined effects of corn-stover-derived biochar modified with ferromanganese on the immobilization and uptake of Cd by rice under continuous aerobic (A), aerobic-flooded (AF), and flooded-aerobic (FA) water management regimes. The results showed that loading iron-manganese significantly increased the maximum sorption capacity (Qm) of Cd on FMBC (50.46 mg g^-1) due to increased surface area, as compared to the pristine biochar (BC, 31.36 mg g^-1). The results revealed that soil Eh and pH were significantly affected by FMBC and it's synergistic application with different water regimes, thus causing significant differences in the concentrations of DTPA-extractable Cd under different treatments. The lowest DTPA-extractable Cd content (0.28-0.46 mg^-1) was observed in the treatment with FMBC (2.5 %) combined FA water amendment, which reduced the content of available Cd in soil by 2.63-28.4 %. Moreover, the treatments with FMBC-FA resulted the proportion of residual Cd increased by 22.2 % compared to the control. Variations in the content and fraction of Cd had a significant influence on its accumulation in the rice grains. The FMBC-FA treatments reduced the Cd concentration in roots, shoots and grains by 37.97 %, 33.98 %, and 53.66 %, respectively, when compared with the control. Predominantly because of the reduction in Cd biological toxicity and the improved soil nutrient content, the combined application increased the biomass and yield of rice to some extent. Taken together, the combination of the Fe-Mn modified biochar and flooded-aerobic water management may potentially be applied in Cd-polluted soil to mitigate the impacts of Cd on rice production.

Water Management Impacts on Chromium Behavior and Uptake by Rice in Paddy Soil with High Geological Background Values

Chromium (Cr) is an expression toxic metal and is seriously released into the soil environment due to its extensive use and mining. Basalt is an important Cr reservoir in the terrestrial environment. Cr in paddy soil can be enriched by chemical weathering. Therefore, basalt-derived paddy soils contain extremely high concentrations of Cr and can enter the human body through the food chain. However, the water management conditions' effect on the transformation of Cr in basalt-derived paddy soil with high geological background values was less recognized. In this study, a pot experiment was conducted to investigate the effects of different water management treatments on the migration and transformation of Cr in a soil-rice system at different rice growth stages. Two water management treatments of continuous flooding (CF) and alternative wet and dry (AWD) and four different rice growth stages were set up. The results showed that AWD treatment significantly reduced the biomass of rice and promoted the absorption of Cr in rice plants. During the four growth periods, the root, stem and leaf of rice increased from 11.24-16.11 mg kg^-1, 0.66-1.56 mg kg^-1 and 0.48-2.29 mg kg^-1 to 12.43-22.60 mg kg^-1, 0.98-3.31 mg kg^-1 and 0.58-2.86 mg kg^-1, respectively. The Cr concentration in roots, stems and leaves of AWD treatment was 40%, 89% and 25% higher than CF treatment in the filling stage, respectively. The AWD treatment also facilitated the potential bioactive fractions conversion to the bioavailable fraction, compared with the CF treatment. In addition, the enrichment of iron-reducing bacteria and sulfate-reducing bacteria with AWD treatment also provided electron iron for the mobilization of Cr, thus affecting the migration and transformation of Cr in the soil. We speculated that the reason for this phenomenon may be the bioavailability of Cr was affected by the biogeochemical cycle of iron under the influence of alternating redox. This indicates that AWD treatment may bring certain environmental risks in contaminated paddy soil with high geological background, and it is necessary to be aware of this risk when using water-saving irrigation to plant rice.

Low levels of arsenic and cadmium in rice grown in southern Florida Histosols - Impacts of water management and soil thickness

Rice is planted as a rotation crop in the sugarcane-dominant Everglades Agricultural Area (EAA) in southern Florida. The Histosols in this area are unlike other mineral soils used to grow rice due to the high organic content and land subsidence caused by rapid oxidation of organic matter upon drainage. It remains unknown if such soils pose a risk of arsenic (As) or cadmium (Cd) mobilization and uptake into rice grain. Both As and Cd are carcinogenic trace elements of concern in rice, and it is important to understand their soil-plant transfer into rice, a staple food of global importance. Here, a mesocosm pot study was conducted using two thicknesses of local soil, deep (D, 50 cm) and shallow (S, 25 cm), under three water managements, conventional flooding (FL), low water table (LWT), and alternating wetting and drying (AWD). Rice was grown to maturity and plant levels of As and Cd were determined. Regardless of treatments, rice grown in these Florida Histolsols has very low Cd concentrations in polished grain (1.5-5.6 μg kg^-1) and relatively low total As (35-150 μg kg^-1) and inorganic As (35-87 μg kg^-1) concentrations in polished grain, which are below regulatory limits. This may be due to the low soil As and Cd levels, high soil cation exchange capacity due to high soil organic matter content, and slightly alkaline soil pH. Grain As was significantly affected by water management (AWD < FL = LWT) and its interaction effect with soil thickness (AWD-D ≤ AWD-S ≤ FL-D = LWT-S = LWT-D ≤ FL-S), resulting in as much as 62 % difference among treatments. Grain Cd was significantly affected by water management (AWD > FL > LWT) without any soil thickness impact. In conclusion, even though water management has more of an impact on rice As and Cd than soil thickness, the low concentrations of As and Cd in rice pose little health risk for consumers.

Decreasing arsenic in rice: Interactions of soil sulfate amendment and water management

Accumulation of inorganic arsenic (iAs) and dimethylarsenate (DMA) in rice threatens human health and rice yield, respectively. We studied the yet unclear interactions of soil sulfate amendment and water management for decreasing As accumulation in rice grain in a pot experiment. We show that soil sulfate amendment (+200 mg S/kg soil) decreased grain iAs by 44% without clearly increasing grain DMA under intermittent flooding from booting stage to maturation. Under continuous flooding during this period, sulfate amendment decreased grain iAs only by 25% but increased grain DMA by 68%. The mechanisms of sulfate amendment effects on grain iAs were not explained by porewater composition or in-planta As sequestration but were allocated to the rhizosphere. Grain iAs closely correlated with As in the root iron-plaque (r = 0.92) which was effectively decreased by sulfate amendment and may have acted as an iAs source for rice uptake. Although both sulfate amendment and intermittent flooding substantially increased porewater DMA concentrations, it was the continuous flooding, irrespective of sulfate amendment, that resulted in rice straighthead disease with 47-55% less yield and 258-320% more DMA in grains than intermittent flooding. This study suggests that combining soil sulfate amendment and intermittent flooding can help to secure the quantity and quality of rice produced in As-affected areas. Our results also imply the key role of rhizosphere processes in controlling both iAs and DMA accumulation in rice which should be elucidated in the future.

Effects of topdressing silicon fertilizer at key stages on uptake and accumulation of arsenic in rice

The booting stage and filling stage have been considered as the key stages for arsenic (As) uptake in rice. In this study, a field study was conducted to investigate the influence of the topdressing different amounts of silicon (Si) fertilizer at the key stages on rice As uptake and accumulation. The results showed that topdressing of a low amount of Si fertilizer at the booting stage and filling stage could increase rice yield, promote the formation of iron plaque and the retention of As on iron plaque, and reduce inorganic As content in brown rice. Compared with the control, the rice grain yield was increased by 22.60% with the topdressing of 20 kg·hm^-2 Si fertilizer at the grain filling stage. As compared with the control, the Fe and As content in iron plaque under the topdressing of 20 kg·hm^-2 Si fertilizer at the booting stage and filling stage was significantly (p < 0.05) increased by 84.34% and 87.78% (Fe content) and 70.96% and 63.80% (As content), respectively. Meanwhile, contents of As in rice roots, stems, and husks at the topdressing of 20 kg·hm^-2 Si fertilizer at the booting stage were significantly (p < 0.05) reduced by 45.10%, 33.34%, and 31.23%, respectively, relative to the control. The lowest inorganic As content (0.21 mg·kg^-1) in brown rice was obtained at the topdressing of 20 kg·hm^-2 Si fertilizer at the booting stage, which was close to the National Food Limit Standard of 0.20 mg·kg^-1 (GB 2762-2017). Therefore, topdressing of 20 kg·hm^-2 Si fertilizer at the booting stage might be considered as an effective method to reduce inorganic As content in brown rice.

Enhanced oxidation and stabilization of arsenic in a soil-rice system by phytosynthesized iron oxide nanomaterials: Mechanistic differences under flooding and draining conditions

Despite arsenic (As) bioavailability being highly correlated with water status and the presence of iron (Fe) minerals, limited information is currently available on how externally applied Fe nanomaterials in soil-rice systems affect As oxidation and stabilization during flooding and draining events. Herein, the stabilization of As in a paddy soil by a phytosynthesized iron oxide nanomaterials (PION) and the related mechanism was investigated using a combination of chemical extraction and functional microbe analysis in soil at both flooding (60 d) and draining (120 d) stages. The application of PION decreased both specifically bound and non-specifically bound As. The As content in rice root, stem, husk and grain was reduced by 78.5, 17.3, 8.4 and 34.4%, respectively, whereas As(III) and As(V) in root declined by 96.9 and 33.3% for the 1% PION treatment after 120 d. Furthermore, the 1% PION treatment decreased the ratio of As(III)/As(V) in the rhizosphere soil, root and stem. Although PION had no significant effect on the overall Shannon index, the distribution of some specific functional microbes changed dramatically. While no As(III) oxidation bacteria were found at 60 d in any treatments, PION treatment increased As(III) oxidation bacteria by 3-9 fold after 120 d cultivation. Structural equation model analysis revealed that the ratio of Fe(III)/Fe(II) affected As stabilization directly at the flooding stage, whereas nitrate reduction and As(III) oxidation microbial groups played a significant role in the stabilization of As at the draining stage. These results highlight that PION exhibits a robust ability to reduce As availability to rice, with chemical oxidation, reduction inhibition and adsorption dominating at the flooding stage, while microbial oxidation, adsorption and coprecipitation dominant during draining.

Silicon nanoparticles in higher plants: Uptake, action, stress tolerance, and crosstalk with phytohormones, antioxidants, and other signalling molecules

Silicon is absorbed as uncharged mono-silicic acid by plant roots through passive absorption of Lsi1, an influx transporter belonging to the aquaporin protein family. Lsi2 then actively effluxes silicon from root cells towards the xylem from where it is exported by Lsi6 for silicon distribution and accumulation to other parts. Recently, it was proposed that silicon nanoparticles (SiNPs) might share a similar route for their uptake and transport. SiNPs then initiate a cascade of morphophysiological adjustments that improve the plant physiology through regulating the expression of many photosynthetic genes and proteins along with photosystem I (PSI) and PSII assemblies. Subsequent improvement in photosynthetic performance and stomatal behaviour correspond to higher growth, development, and productivity. On many occasions, SiNPs have demonstrated a protective role during stressful environments by improving plant-water status, source-sink potential, reactive oxygen species (ROS) metabolism, and enzymatic profile. The present review comprehensively discusses the crop improvement potential of SiNPs stretching their role during optimal and abiotic stress conditions including salinity, drought, temperature, heavy metals, and ultraviolet (UV) radiation. Moreover, in the later section of this review, we offered the understanding that most of these upgrades can be explained by SiNPs intricate correspondence with phytohormones, antioxidants, and signalling molecules. SiNPs can modulate the endogenous phytohormones level such as abscisic acid (ABA), auxins (IAAs), cytokinins (CKs), ethylene (ET), gibberellins (GAs), and jasmonic acid (JA). Altered phytohormones level affects plant growth, development, and productivity at various organ and tissue levels. Similarly, SiNPs regulate the activities of catalase (CAT), ascorbate peroxidase (APX), superoxide dismutase (SOD), and ascorbate-glutathione (AsA-GSH) cycle leading to an upgraded defence system. At the cellular and subcellular levels, SiNPs crosstalk with various signalling molecules such as Ca²⁺, K⁺, Na⁺, nitric oxide (NO), ROS, soluble sugars, and transcription factors (TFs) was also explained.

Arsenic accumulation in rice: Alternative irrigation regimes produce rice safe from arsenic contamination

The natural occurrence of arsenic (As) in groundwater & soils and its bioaccumulation in rice grains is a major health concern worldwide. To combat the problem, best combination of irrigation management and suitable rice variety altering As content in grains must be ensured. With this aim, a field trial was conducted with two rice varieties and water management including alternate wetting and drying (AWD) and continuous flooding (CF) irrigation regimes with As contaminated groundwater (AsW) and temporarily stored groundwater (TSG) and river water for only CF (as control). Results revealed that As content in different portions of paddy plant was significantly different (P < 0.001) with irrigation practices and rice varieties. AWD irrigation with TSG accumulated lower As in rice grains compared with CF-AsW for both varieties. Data showed that AWD-TSG practice led to 61.37% and 60.34% grain As reduction for BRRI dhan28 and BRRI dhan29, respectively, compared with CF-AsW. For Principle Component Analysis (PCA), first principle component (PC1) explained 91.7% of the variability and irrigation water As, soil total and available As, straw As, root As and husk As were the dominating parameters. With significant (P < 0.05) variation in yields between the genotypes, AWD increased grain yield by 29.25% in BRRI dhan29 Compared with CF. However, translocation factor (TF) and bioconcentration factor (BCF) for both varieties were less than one for all the treatments. The addition of this study to our knowledge base is that, AWD-TSG with BRRI dhan29 can be an As-safe practice without compromising yields.

Transformation of arsenic species by diverse endophytic bacteria of rice roots

Rice growing in flooded paddy soil often accumulates considerable levels of inorganic and organic arsenic (As) species, which may cause toxicity to plants and/or pose a risk to human health. The bioavailability and toxicity of As in soil depends on its chemical species, which undergo multiple transformations driven primarily by soil microbes. However, the role of endophytes inside rice roots in As species transformation remains largely unknown. We quantified the abundances of microbial functional genes involved in As transformation in the endosphere and rhizosphere of rice roots growing in three paddy soils in a pot experiment. We also isolated 46 different bacterial endophytes and tested their abilities to transform various As species. The absolute abundances of the arsenate reductase gene arsC and the dissimilatory arsenate reductase gene arrA in the endosphere were comparable to those in the rhizosphere, whereas the absolute abundances of the arsenite methylation gene arsM and arsenite oxidation gene aioA in the endosphere were lower. After normalization based on the bacterial 16S rRNA gene, all four As transformation genes showed higher relative abundances in the endosphere than in the rhizosphere. Consistent with the functional gene data, all of the 30 aerobic endophytic isolates were able to reduce arsenate, but only 3 strains could oxidize arsenite. Among the 16 anaerobic endophytic isolates, 4 strains belonging to Desulfovibrio, Terrisporobacter or Clostridium could methylate arsenite and/or methylarsenite. Six strains of aerobic endophytes could demethylate methylarsenite, among which three strains also could reduce and demethylate methylarsenate. None of the isolates could demethylate dimethylarsenate. These results suggest that diverse endophytes living inside rice roots could participate in As species transformation and affect As accumulation and species distribution in rice plants.

Double-edged effects of elevating temperature on the aging of exogenous arsenic in flooded paddy soils

Temperature variation can have a significant impact on arsenic (As) bioavailability in paddy soils. However, details regarding the transformation of exogenous As during the aging process in paddy soils at various temperatures remain unclear. This work investigated the effects of temperature on the As extractability and As species transformation of three paddy soils spiked with exogenous arsenate at 60 mg kg^-1 under flooded aging and explored the related chemical and microbial mechanisms. The results showed that 0.05 M NH₄H₂PO₄-extractable As decreased over time during flooded aging for 192 days, and it decreased by approximately one-third at 35 °C compared with 15 °C and 25 °C at the same aging time, indicating that higher temperatures facilitated the decrease in As extractability. As(V) reduction mainly occurred at 35 °C because the abundance and As(V)-reducing capacity of the predominant indigenous bacteria, the Bacillus sp strains, and the abundance of the arrA gene were significantly higher than those at 15 °C and 25 °C. The reduction of As(V) to As(III) and aging occurred simultaneously. The kinetic models were established, and the rate constants of the reduction and aging processes were obtained. Soil properties significantly affected the aging and reduction processes of extractable As(V). Our study indicated that elevating temperature had dual effects on the environmental risk of As in the flooded aging process. The previous definition of "aging" based on cationic metals needs to be updated according to the transformation characteristics of As species in flooded conditions. Our results addressed the necessity of impeding the reduction of As(V) in paddy soils under global warming.

Antimony speciation, phytochelatin stimulation and toxicity in plants

Antimony (Sb) is a toxic metalloid that has been listed as a priority pollutant. The environmental impacts of Sb have recently attracted attention, but its phytotoxicity and biological transformation remain poorly understood. In this study, Sb speciation and transformation in plant roots was quantified by Sb K-edge X-ray absorption spectroscopy. In addition, the phytotoxicity of antimonate (Sb^V) on six plant species was assessed by measuring plant photosynthesis, growth, and phytochelatin production induced by Sb^V. Linear combination fitting of the Sb K-edge X-ray absorption near-edge structure (XANES) spectra indicated reduction of Sb^V was limited to ∼5-33% of Sb. The data confirmed that Sb-polygalacturonic acid was the predominant chemical form in all plant species (up to 95%), indicating Sb was primarily bound to the cell walls of plant roots. Shell fitting of Sb K-edge X-ray absorption fine-structure (EXAFS) spectra confirmed Sb-O and Sb-C were the dominant scattering paths. The fitting indicated that Sb^V was bound to hydroxyl functional groups of cell walls, via development of a local coordination environment analogous to Sb-polygalacturonic acid. This is the first study to demonstrate the key role of plant cell walls in Sb metabolism.

Effects and Mechanisms of Copper Oxide Nanoparticles with Regard to Arsenic Availability in Soil-Rice Systems: Adsorption Behavior and Microbial Response

Copper oxide nanoparticles (CuO NPs) are widely used as fungicides in agriculture. Arsenic (As) is a ubiquitous contaminant in paddy soil. The present study was focused on the adsorption behavior of CuO NPs with regard to As as well as the characteristics of the microbial community changes in As-contaminated soil-rice systems in response to CuO NPs. The study found that CuO NPs could be a temporary sink of As in soil; a high dose of CuO NPs promoted the release of As from crystalline iron oxide, which increased the As content in the liquid phase. The study also found that the As bioavailability changed significantly when the dose of CuO NPs was higher than 50 mg kg^-1 in the soil-rice system. The addition of 100 mg kg^-1 CuO NPs increased the microbial diversity and the abundance of genes involved in As cycling, decreased the abundance of Fe(III)-reducing bacteria and sulfate-reducing genes, and decreased As accumulation in grains. Treatment with 500 mg kg^-1 CuO NPs increased the abundance of Fe(III)-reducing bacteria and sulfate-reducing genes, decreased Fe plaques, and increased As accumulation in rice. The adverse effects of CuO NPs on crops and associated risks need to be considered carefully.

Identification of Soil Arsenic Contamination in Rice Paddy Field Based on Hyperspectral Reflectance Approach

Toxic heavy metals in soil negatively impact soil’s physical, biological, and chemical characteristics, and also human wellbeing. The traditional approach of chemical analysis procedures for assessing soil toxicant element concentration is time-consuming and expensive. Due to accessibility, reliability, and rapidity at a high temporal and spatial resolution, hyperspectral remote sensing within the Vis-NIR region is an indispensable and widely used approach in today’s world for monitoring broad regions and controlling soil arsenic (As) pollution in agricultural land. This study investigates the effectiveness of hyperspectral reflectance approaches in different regions for assessing soil As pollutants, as well as a basic review of space-borne earth observation hyperspectral sensors. Multivariate and various regression models were developed to avoid collinearity and improve prediction capabilities using spectral bands with the perfect correlation coefficients to access the soil As contamination in previous studies. This review highlights some of the most significant factors to consider when developing a remote sensing approach for soil As contamination in the future, as well as the potential limits of employing spectroscopy data.

Optimal soil Eh, pH for simultaneous decrease of bioavailable Cd, As in co-contaminated paddy soil under water management strategies

The co-contamination with cadmium (Cd) and arsenic (As) in the paddy soil is the most seriously combined pollution of toxic elements in China, and it is rather difficult to decrease bioavailable Cd and As levels in soil because of the opposite ionic forms of bioavailable Cd (cation) and As (anion). This study explored the optimal conditions of Eh and pH in different soils for simultaneous decrease of Cd and As bioavailabilities in the soil-rice system through soil culture and rice pot experiments under water management strategies. The results showed that near neutral soil pH (7.0) were eventually observed under long-term flooding conditions. Under unflooded conditions, soil pH is the dominant factor influencing bioavailabilities of Cd and As, while under flooded conditions, Eh becomes the most important factor. Pot experiments showed that flooding significantly reduced the Cd concentration in rice grains from 54.5% to 95.5%, but concomitantly increased rice As concentration substantially (214%-302%). By evaluating the trade-off value between the bioavailabilities of Cd and As in the soil, the minimal trade-off value was obtained when the soil Eh was -130 mV and the pH was 6.8.

The relative contributions of root uptake and remobilization to the loading of Cd and As into rice grains: Implications in simultaneously controlling grain Cd and As accumulation using a segmented water management strategy

Cadmium (Cd) and arsenic (As) are loaded into rice grain via two pathways: i) root uptake from the soil and then translocation to the grain, and ii) remobilization of Cd and As previously accumulated within the vegetative tissues to the grain. However, the relative contributions of the two pathways are not well understood in soil-grown rice plants. In this study, we used eight different water management regimes applied at different growth periods to manipulate the concentrations of Cd and As in porewater and then established a mathematical model to estimate the relative importance of the two pathways. Different water management regimes had dramatic and opposite effects on the solubility of Cd and As in soil, and their subsequent accumulation in both straw and grain. Water management applied at different growth periods had markedly different impacts on grain Cd and As concentrations. Water management during grain filling had a much greater impact on grain Cd than on grain As concentrations, whereas water treatment during the vegetative growth stage had a larger effect on grain As concentrations. Under the typical water management practice (i.e. flooding through the vegetative stage followed by drainage during grain filling), grain filling is the key period for the accumulation of Cd in the grain, with 98% of the grain Cd from root uptake during this period and the contribution of remobilization being very limited. In contrast, 95% of the grain As was remobilized from that accumulated within the plant prior to the grain filling, with the tillering, jointing, and heading period each contributing 20-40% of the grain As, whereas root uptake during grain filling contributed minor. These differences can be harnessed to design a segmented water management strategy to control grain Cd and As accumulation simultaneously.

Potential use of arbuscular mycorrhizal fungi for simultaneous mitigation of arsenic and cadmium accumulation in rice

Rice polluted by metal(loid)s, especially arsenic (As) and cadmium (Cd), imposes serious health risks. Numerous studies have demonstrated that the obligate plant symbionts arbuscular mycorrhizal fungi (AMF) can reduce As and Cd concentrations in rice. The behaviours of metal(loid)s in the soil-rice-AMF system are of significant interest for scientists in the fields of plant biology, microbiology, agriculture, and environmental science. We review the mechanisms of As and Cd accumulation in rice with and without the involvement of AMF. In the context of the soil-rice-AMF system, we assess and discuss the role of AMF in affecting soil ion mobility, chemical forms, transport pathways (including the symplast and apoplast), and genotype variation. A potential strategy for AMF application in rice fields is considered, followed by future research directions to improve theoretical understanding and encourage field application.

Toxic metals and metalloids: Uptake, transport, detoxification, phytoremediation, and crop improvement for safer food

Agricultural soils are under threat of toxic metal/metalloid contamination from anthropogenic activities, leading to excessive accumulation of arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg) in food crops that poses significant risks to human health. Understanding how these toxic metals and their methylated species are taken up, translocated, and detoxified is prerequisite to developing strategies to limit their accumulation for safer food. Toxic metals are taken up and transported across different cellular compartments and plant tissues via various transporters for essential or beneficial nutrients, e.g. As by phosphate and silicon transporters, and Cd by manganese (Mn), zinc (Zn), and iron (Fe) transporters. These transport processes are subjected to interactions with nutrients and the regulation at the transcriptional and post-translational levels. Complexation with thiol-rich compounds, such as phytochelatins, and sequestration in the vacuoles are the common mechanisms for detoxification and for limiting their translocation. A number of genes involved in toxic metal uptake, transport, and detoxification have been identified, offering targets for genetic manipulation via gene editing or transgenic technologies. Natural variations in toxic metal accumulation exist within crop germplasm, and some of the quantitative trait loci underlying these variations have been cloned, paving the way for marker-assisted breeding of low metal accumulation crops. Using plants to extract and remove toxic metals from soil is also possible, but this phytoremediation approach requires metal hyperaccumulation for efficiency. Knowledge gaps and future research needs are also discussed.

Effects of optimized water management on the uptake and translocation of cadmium and arsenic in Oryza sativa L. in two contaminated soils

Water management has opposite effects on the bioavailability of Cd and As in soil. In order to identify the most efficient water management strategy for reducing Cd and As accumulations and amino acid (AA) synthesis in rice in two soils with different Cd and As contents, a pot experiments were conducted in greenhouse. A treatment consisting of 5 days of flooding followed by 3 days of drainage (F5D3, repeated every 8 days) was identified as the most effective treatment for simultaneously decreasing Cd and As in grains, with reductions of grain Cd and As contents of more than 80.0% and 73.1%, respectively, compared with either a drained treatment or a flooded treatment alone; this is probably related to the high efficiency of the F5D3 treatment in reducing dissolved Cd and As according to its minimum "trade-off value" (an index for evaluating the degree of trade-off between soil solution As and Cd concentrations in water management condition), due to the variations in grain Cd and As contents which were significantly correlated with the variations in soil solution Cd (R2=0.98) and As (R2=0.92, P=0.0001) concentrations. Additionally, grain Cd content was also significantly related to the organs Cd contents (especially root Cd content, R2=0.99) and the root-to-shoot Cd translocation factors (R2=0.99), whereas grain As content was significantly related to soil Eh (R2=-0.82, P=0.003) and pH (R2=0.88, P=0.0008). The AA contents in organs under the F5D3 treatment were lower than those under the flooded and drained treatments. These results indicated that the F5D3 treatment was the most effective water management strategy for simultaneously reducing grain Cd and As contents and AA synthesis in rice, which was probably due to there being no need for rice to synthesize abundant AAs to chelate metal ions.

Arsenic contamination, impact and mitigation strategies in rice agro-environment: An inclusive insight

Arsenic (As) contamination and its adverse consequences on rice agroecosystem are well known. Rice has the credit to feed more than 50% of the world population but concurrently, rice accumulates a substantial amount of As, thereby compromising food security. The gravity of the situation lays in the fact that the population in theAs uncontaminated areas may be accidentally exposed to toxic levels of As from rice consumption. In this review, we are trying to summarize the documents on the impact of As contamination and phytotoxicity in past two decades. The unique feature of this attempt is wide spectrum coverages of topics, and that makes it truly an interdisciplinary review. Aprat from the behaviour of As in rice field soil, we have documented the cellular and molecular response of rice plant upon exposure to As. The potential of various mitigation strategies with particular emphasis on using biochar, seed priming technology, irrigation management, transgenic variety development and other agronomic methods have been critically explored. The review attempts to give a comprehensive and multidiciplinary insight into the behaviour of As in Paddy -Water - Soil - Plate prospective from molecular to post-harvest phase. From the comprehensive literature review, we may conclude that considerable emphasis on rice grain, nutritional and anti-nutritional components, and grain quality traits under arsenic stress condition is yet to be given. Besides these, some emerging mitigation options like seed priming technology, adoption of nanotechnological strategies, applications of biochar should be fortified in large scale without interfering with the proper use of biodiversity.

Setting-up of different water managements as mitigation strategy of the environmental impact of paddy rice

Northern Italy represents the most important rice-growing district in Europe. In this area, rice is the main annual crop and the main revenues source for farmers. However, Italian climatic condition led to a traditional cultivation characterized by continuous flooding, causing emissions of methane into the atmosphere due to the organic matter fermentation in anaerobic conditions, and, consequently, a high environmental impact. The water conditions of paddy fields also affect heavy metals uptake by rice plants. In this context, this study focuses on the evaluation of environmental impact and of heavy metal content in paddy rice, and it may represent an important step in mitigating the environmental impact of rice production. In detail, this study quantifies the environmental benefits related to the adoption of an alternative water management characterized by an additional aeration period during stem elongation. To this purpose, field trials were carried out and the Life Cycle Assessment (LCA) approach was applied with a cradle-to-farm gate perspective. The potential environmental impact of the production of two rice varieties (Carnaroli and Caravaggio) was analysed in terms of 12 different impact categories and dehulled rice grain were analysed for arsenic and cadmium content. Alternative flooding decreases CH₄ emissions in all cases evaluated (from 15% to 52%), resulting in a reduction in the climate change impact of rice cultivation (from 12% to 32%). Furthermore, the alternative water management does not influence grain yield and it reduces all the other environmental impact categories in 2 out of 4 cases. Regarding the heavy metals contents, the arsenic content in the grain decreases in all alternative scenarios, whereas the cadmium content increases, while remaining well below the legal limits.

Niche Differentiation of Arsenic-Transforming Microbial Groups in the Rice Rhizosphere Compartments as Impacted by Water Management and Soil-Arsenic Concentrations

Arsenic (As) bioavailability in the rice rhizosphere is influenced by many microbial interactions, particularly by metal-transforming functional groups at the root-soil interface. This study was conducted to examine As-transforming microbes and As-speciation in the rice rhizosphere compartments, in response to two different water management practices (continuous and intermittently flooded), established on fields with high to low soil-As concentration. Microbial functional gene composition in the rhizosphere and root-plaque compartments were characterized using the GeoChip 4.0 microarray. Arsenic speciation and concentrations were analyzed in the rhizosphere soil, root-plaque, pore water, and grain samples. Results confirmed several As-biotransformation processes in the rice rhizosphere compartments, and distinct assemblage of As-reducing and methylating bacteria was observed between the root-plaque and rhizosphere. Results confirmed higher potential for microbial As-reduction and As-methylation in continuously flooded, long term As-contaminated fields, which accumulated highest concentrations of As^III and methyl-As concentrations in pore water and rice grains. Water management treatment significantly altered As-speciation in the rhizosphere, and intermittent flooding reduced methyl-As and As^III concentrations in the pore water, root-plaque and rice grain. Ordination and taxonomic analysis of detected gene-probes indicated that root-plaque and rhizosphere assembled significantly different microbial functional groups demonstrating niche separation. Taxonomic non-redundancy was evident, suggesting that As-reduction, -oxidation and -methylation processes were performed by different microbial functional groups. It was also evident that As transformation was coupled to different biogeochemical cycling processes (nutrient assimilation, carbon metabolism etc.) in the compartments and between treatments, revealing functional non-redundancy of rice-rhizosphere microbiome in response to local biogeochemical conditions and As contamination. This study provided novel insights on As-biotransformation processes and their implications on As-chemistry at the root-soil interface and their responses to water management, which could be applied for mitigating As-bioavailability and accumulation in rice grains.